Wheel alignment measurement

ABSTRACT

A method of onboard determination of vehicle wheel alignment is provided. In a first step, a lateral acceleration signal is generated, using an accelerometer mounted on the vehicle wheel. In a second step, a first lateral acceleration value a y Acc  is derived from a first portion of the lateral acceleration signal, which is during a longitudinal acceleration a 0  of the vehicle. In a third step, a second lateral acceleration value a y CS  is derived from a second portion of the lateral acceleration signal, which is measured when the longitudinal acceleration has ceased and the vehicle is travelling at constant speed. In a fourth step, the magnitude of the longitudinal acceleration a 0  associated with the first portion of the lateral acceleration signal is measured. In a fifth step, a toe angle α of the vehicle wheel is determined according to the following relationship: 
     
       
         
           
             α 
             = 
             
               
                 arcsin 
                 ( 
                 
                   
                     
                       a 
                       
                         Y 
                         Acc 
                       
                     
                     - 
                     
                       a 
                       
                         Y 
                         CS 
                       
                     
                   
                   
                     
                       - 
                       
                         a 
                         0 
                       
                     
                      
                     
                       cos 
                        
                       
                         ( 
                         
                           arcsin 
                            
                           
                             ( 
                             
                               
                                 a 
                                 
                                   Y 
                                   CS 
                                 
                               
                               g 
                             
                             ) 
                           
                         
                         ) 
                       
                     
                   
                 
                 ) 
               
               · 
               
                 180 
                 π 
               
             
           
         
       
     
     where g is the gravitational acceleration constant.

FIELD OF THE INVENTION

The present invention relates to a method and a system for on-boardmeasurement of vehicle wheel alignment.

BACKGROUND TO THE INVENTION

A common problem found in many trucks today is that their wheels andaxles are incorrectly positioned. This incorrect position is too smallfor the human eye to see and the effects become noticeable only when theproblem has become severe. The incorrect position is called wheelmisalignment. Wheel misalignment leads to increased tire wear and fuelconsumption and decreased control of the vehicle. Wheel misalignment canbe due to components of a new truck setting in, bumps, potholes,suspension fatigue, or loading of the vehicle. It is therefore necessaryto perform regular checks on the health of the vehicle.

If the truck owner decides to correct the problem of wheel misalignment,he has to take the vehicle to a service centre. The correction is donewhile the vehicle is at a standstill and without the extra weight of acargo. The position of the wheel is then measured using alignmentequipment. The position of the wheel is then altered by a techniciantill the values are within the recommended settings. Companies thatperform wheel alignment commonly have a yearly check-up of their fleet.Midway the year, the truck wheels might already be misaligned, while thesymptoms are too small to be noticed. This means that the truck mightneed to wait up to a year to be corrected.

This brings the need for onboard measurement equipment or dynamicalignment, to enable fleet operators to implement more effectivemaintenance programmes. The misalignment will be detected in an earlyphase, before causing too much damage to the tire. Dynamic alignmenttakes into consideration the loading pattern and dynamic behaviour ofthe vehicle whilst on the road, giving a more accurate measurement ofthe alignment angles.

SUMMARY OF THE INVENTION

The present invention resides in a method of onboard determination ofvehicle wheel alignment. The method comprises a first step of generatinga lateral acceleration signal using an accelerometer mounted on thevehicle wheel.

Preferably, the accelerometer is a three-axis accelerometer that ismounted on a side face of the wheel rim. It is also possible to use atwo-axis accelerometer. The accelerometer is preferably mounted close tothe centre of the wheel rim side face, to minimise the centrifugalforces acting on the accelerometer.

Suitably, the method further comprises a step of signal processing inwhich misalignment errors are removed and noise is filtered from thesignal. Furthermore, as will be clear to the skilled person, theaccelerometer is preferably calibrated after being mounted to thevehicle wheel.

In a second step, a first lateral acceleration value a_(y Acc) isobtained from a first portion of the lateral acceleration signal. Thefirst portion of the signal is measured during a period when the vehicleis accelerating in a longitudinal direction.

In a third step, a second lateral acceleration value a_(y CS) isobtained from a second portion of the lateral acceleration signal. Thesecond portion is measured when the longitudinal acceleration has ceasedand the vehicle is travelling at constant speed.

In a fourth step, the magnitude a₀ of the longitudinal acceleration,associated with the first portion of the lateral acceleration signal, ismeasured. The longitudinal acceleration can be measured using the sameaccelerometer mounted on the wheel rim, or using a separateaccelerometer that is mounted elsewhere on the vehicle, e.g. on thevehicle chassis.

In a fifth step, a toe angle α of the vehicle wheel is calculatedaccording to the following relationship:

$\alpha = {{\arcsin\left( \frac{a_{Y_{Acc}} - a_{Y_{CS}}}{{- a_{0}}{\cos \left( {\arcsin \left( \frac{a_{Y_{CS}}}{g} \right)} \right)}} \right)} \cdot \frac{180}{\pi}}$

where g is the gravitational acceleration constant.

In a sixth step, the camber angle γ of the vehicle wheel can also becalculated, using the following relationship:

$\gamma = {{\arcsin \left( {- \frac{a_{Y_{CS}}}{g}} \right)} \cdot {\frac{180}{\pi}.}}$

When the calculated value of the toe angle or the camber angle fallsoutside of prescribed allowable values, an alert is generated. A fleetowner is thus able to schedule maintenance when it is truly required.

BRIEF DESCRIPTION OF THE DRAWINGS

The theory behind the method of the invention will now be explained ingreater detail, with reference to the detailed description and to thedrawings, in which:

FIGS. 1 a and 1 b schematically depict part of a vehicle in which thefront wheels are shown in a toe-in position and in a toe-out positionrespectively;

FIG. 1 c shows a vehicle wheel with a varying camber angle;

FIG. 2 is a flowchart of an embodiment of the method according to theinvention;

FIG. 3 shows a plot of a lateral acceleration signal measured during aperiod of longitudinal vehicle acceleration followed by a period ofvehicle constant speed.

DETAILED DESCRIPTION

To ensure that desired characteristics are obtained while driving, thetoe and camber angles of vehicle wheels may be set to non-zero values.Toe angle is given by the difference in distance between the leading andtrailing edges of a wheel set. In FIGS. 1 a and 1 b, the front wheels11, 12 of a passenger vehicle 10 are shown. In FIG. 1 a, a distanced_(L) between the leading edges of the right 11 and left 12 front wheelsis smaller than a distance d_(T) between the trailing edges, which isreferred to as a toe-in position. Correspondingly, each wheel has a toeangle α, representing the angle to which the wheel is out of parallelwith a longitudinal direction of the vehicle. Wheels in a toe-inposition have a positive toe angle value +α.

The vehicle front wheels 11, 12 are depicted in a toe-out position inFIG. 1 b, whereby the distance d_(L) between the leading edges isgreater than the distance d_(T) between the trailing edges.Correspondingly, each wheel has a negative toe angle value −α.

During driving of a heavy truck, for example, a toe angle of zero isadvantageous. But when the vehicle is moving, the wheels tend totoe-out. A slight toe-in setting (positive toe angle) of the wheelstherefore enables to compensate for this. By ensuring that the vehiclewheels remain substantially at their optimal toe angle, tyre wear can beminimized and fuel consumption optimized, which is particularlyimportant for the operators of commercial fleets. The present inventionprovides an onboard method of determining and monitoring toe angle, sothat unacceptable deviations can be detected early.

The inventive method comprises measurement of wheel lateralacceleration, and is based on the understanding that detecting changesin toe angle relies on forward acceleration of the vehicle. Withreference to FIG. 1 a, let us assume that the vehicle 10 is acceleratingin a forward direction. When the vehicle accelerates, the accelerationvector a₀ is broken down into two components at each wheel:

-   -   a longitudinal acceleration a_(x) in the longitudinal direction        of the wheel 11, 12, which is responsible for the rotation of        the wheel (when no longitudinal slip is present); and    -   a lateral acceleration ay in the lateral direction of the wheel        11, 12, which causes slip.

The decomposition of the vehicle acceleration a₀ into longitudinalacceleration a_(x) and lateral acceleration a_(y) components is largelydetermined by the toe angle.

The greater the toe angle α, the greater the lateral component duringacceleration. Wheel camber angle also plays a role, however, and must betaken into account.

As shown in FIG. 1 c, which is a front view of the front left wheel 12of the vehicle of FIGS. 1 a and 1 b, camber angle γ is the angle of thewheel 12 relative to vertical, as viewed from the front or the rear ofthe car. If the wheel leans in towards the chassis, it has negativecamber; if it leans away from the car, it has positive camber.

In a truck, for example, the purposes of camber are twofold: todistribute the weight of the vehicle evenly on the tires and provideeasy steering, by allowing the weight of the vehicle to be carried bythe inner wheel bearing and spindle.

Too much or too little camber will cause tire wear and wear on the balljoints and bearings. Furthermore in the event of unequal camber, thevehicle will pull to the side with the greater camber. The method of thepresent invention further enables onboard monitoring of wheel camberangle.

The method comprises a step of measuring wheel lateral accelerationduring vehicle acceleration. This may be done using e.g. anaccelerometer that is mounted to a wheel rim of the wheel.

The general formula for deriving toe and camber from lateralacceleration is given by:

a_(Y) =−g.sin γ−a ₀.sin α.cos γ  [Equation 1]

Where

Parameter Definition Unit a_(y) Lateral acceleration m/s² gGravitational acceleration m/s² γ Camber angle degrees α Toe angledegrees a₀ Vehicle acceleration in m/s² longitudinal direction

The method further comprises a step of measuring the vehicleacceleration a₀. This may be done using the same accelerometer mountedon the wheel rim, or may be measured at a different location on thevehicle.

By measuring a₀, Equation 1 then contains two unknowns: the camber angleγ and the toe angle α. Determination of the camber and toe anglesaccording to the method of the invention lies in understanding thesubtle lateral acceleration differences found during and after anacceleration manoeuvre. The camber angle is ever present in themeasurement of lateral acceleration, as it depends on the constantgravitational acceleration. The toe angle, however, only reveals itselfduring an acceleration of the vehicle. It is therefore possible to solveEquation 1 for α and γ, by measuring the lateral acceleration at twoinstances.

The first instance is when the vehicle is accelerating, and thecorresponding first lateral acceleration measurement will be referred toan accelerating lateral acceleration a_(y Acc).

The second instance is when acceleration has ceased and the vehicle istravelling at constant speed. The corresponding second lateralacceleration measurement will be referred to as a constant speed lateralacceleration, a_(y CS).

When the vehicle is travelling at constant speed, a₀=0, and Equation 1becomes:

a _(Y) =−g.sin γ

The lateral acceleration can be assumed to equal the second lateralacceleration measurement, a_(y CS), meaning that the camber angle γ maybe calculated according to the following relationship:

$\begin{matrix}{\gamma = {{\arcsin \left( {- \frac{a_{Y_{cs}}}{g}} \right)} \cdot \frac{180}{\pi}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

During vehicle acceleration, the lateral acceleration a_(y) can beassumed to equal the first lateral acceleration measurement a_(y Acc).The toe angle α can therefore be calculated by substituting Equation 2into Equation 1, to obtain the following relationship:

$\begin{matrix}{\alpha = {{\arcsin\left( \frac{a_{Y_{CS}} - a_{Y_{Acc}}}{{- a_{0}}{\cos \left( {\arcsin \left( \frac{a_{Y_{Acc}}}{g} \right)} \right)}} \right)} \cdot \frac{180}{\pi}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

The method of the invention is depicted in the flowchart of the FIG. 2.In a first step 21, wheel lateral acceleration is measured using anaccelerometer as described above, which generates a lateral accelerationsignal. In practice, the accelerometer mounted on the wheel will measurenot only the wheel accelerations, but also crosstalk due to themisalignment of the sensor on the wheel and the errors within theaccelerometer. The accelerometer will also pick up noise coming fromengine vibration, road vibration, wind and other outside factors.

The method therefore suitably comprises a step 21A of processing thelateral acceleration signal to remove crosstalk and/or misalignmenterrors.

In one embodiment, an algorithm is used to remove crosstalk.

The crosstalk error in a three-axis accelerometer comes from sensing theacceleration of other axes, which can be due to internal voltage effectsof the accelerometers or other physical properties of MEMS-type devices.The accelerations with crosstalk are given by:

a _(X cross) =a _(X)+crosstalk_(XY) ·α _(Y)+crosstalk_(XZ) ·a _(Z)

a _(Y cross) =a _(Y)+crosstalk_(XY) ·α _(X)+crosstalk_(YZ) ·a _(Z)

a _(z cross) =a _(z)+crosstalk_(XZ) ·α _(X)+crosstalk_(YZ) ·a _(Y)

Where crosstalk_(XY), crosstalk_(XZ), crosstalk_(YZ) are thepercentages. Typical crosstalk errors lie between 1-2 percent.

Solving for the original signal gives the following equation for thelateral acceleration signal:

$a_{Y} = {\frac{\left( {a_{X_{cross}} \cdot \left( {{crosstalk}_{XY} - {{crosstalk}_{XZ} \cdot {crosstalk}_{YZ}}} \right)} \right)}{\begin{pmatrix}{{crosstalk}_{XY}^{2} - {2 \cdot {crosstalk}_{XY} \cdot {crosstalk}_{XZ} \cdot}} \\{{crosstalk}_{YZ} + {crosstalk}_{XZ}^{2} + {crosstalk}_{YZ}^{2} - 1}\end{pmatrix}} + \frac{\left( {a_{z_{cross}} \cdot \left( {{crosstalk}_{YZ} - {{crosstalk}_{XY} \cdot {crosstalk}_{XZ}}} \right)} \right)}{\begin{pmatrix}{{crosstalk}_{XY}^{2} - {2 \cdot {crosstalk}_{XY} \cdot {crosstalk}_{XZ} \cdot}} \\{{crosstalk}_{YZ} + {crosstalk}_{XZ}^{2} + {crosstalk}_{YZ}^{2} - 1}\end{pmatrix}} + \frac{\left( {a_{Y_{cross}} \cdot \left( {{crosstalk}_{XZ} - 1} \right)} \right)}{\begin{pmatrix}{{crosstalk}_{XY}^{2} - {2 \cdot {crosstalk}_{XY} \cdot {crosstalk}_{XZ} \cdot}} \\{{crosstalk}_{YZ} + {crosstalk}_{XZ}^{2} + {crosstalk}_{YZ}^{2} - 1}\end{pmatrix}} +}$

The terms crosstalk_(XY), crosstalk_(XZ), crosstalk_(YZ) can bedetermined through experimentation. This removes the crosstalk X(longitudinal) and Z (vertical) acceleration terms, to obtain aprocessed lateral acceleration signal that is largely error free.

Alternatively, depending on the accelerometer used, the internalcrosstalk effects may be negligible, in which case it is sufficient todetermine a constant offset during the calibration of the sensor, whichoffset is then subtracted from the lateral acceleration signal.

The processed signal now contains part acceleration data and part noise.The method suitably further comprises a further step 21B of filteringthe processed signal to remove the noise. The important accelerationdata has an almost constant value and thus has a very low frequency; thenoise has a high frequency. By introducing a low pass filter, the noisewill be filtered out to an acceptable extent.

An example of a filtered lateral acceleration signal is shown in FIG. 3.The signal comprises a first portion 31 and a second portion 32. Thefirst portion 31 is obtained while the vehicle is accelerating in aforward direction; the second portion 32 is obtained immediately afterthe acceleration has ceased, while the vehicle is moving at constantspeed. The first and second portions are separated by a distinct jump.In this example, a 4^(th) order Butterworth filter with cutoff frequency0.01 Hz was used. The Butterworth filter has a very flat frequencyresponse, which is useful in the sharp jump that occurs after theacceleration stops. It is also possible to use other types of filter andother cutoff frequency values.

The accelerating lateral acceleration a_(y Acc), is obtained from thefirst portion 31 of the signal in a second step 22 of the inventivemethod (refer FIG. 2) and the constant speed lateral accelerationa_(y CS) is obtained from the second portion 32 of the signal in a thirdstep 23 of the method. Suitably, steps 22 and 23 of the method compriseaveraging the first and second portions of the signals in order toobtain and average values for a_(y Acc) and a_(y CS) respectively.

As mentioned before, the method further comprises a fourth step 24 ofmeasuring the magnitude of the forward acceleration a₀ of the vehicle.

In a fifth step 25, toe angle is calculated from the average values fora_(y Acc) and a_(y CS) and from the magnitude of the forwardacceleration a₀, using Equation 3.

Suitably, the preceding steps of the method are repeated in order toobtain a series of toe angle values α₁, α₂, α₃ . . . and the methodcomprises a further step 27 in which the series of toe angle values areaveraged, to obtain an average toe angle value.

The method of the invention may also comprise a sixth step 26 ofcalculating camber angle from the constant speed lateral accelerationa_(y CS), obtained in step 23. Again, the preceding steps of the methodare suitably repeated, to obtain a series of camber angle values γ₁, γ₂,γ₃ . . . and the method may comprise a further step 28 of calculating anaverage camber angle from the series γ₁, γ₂, γ₃ . . . .

Thus, the method of the invention provides a straightforward way ofonboard determination of wheel toe angle and wheel camber angle,enabling early detection of misalignment.

A number of aspects/embodiments of the invention have been described. Itis to be understood that each aspect/embodiment may be combined with anyother aspect/embodiment. Moreover the invention is not restricted to thedescribed embodiments, but may be varied within the scope of theaccompanying patent claims.

1. A method of onboard determination of vehicle wheel alignment, themethod comprising: a first step of measuring lateral acceleration, usingan accelerometer mounted on the vehicle wheel, to obtain a lateralacceleration signal; a second step of obtaining a first lateralacceleration value a_(y Acc) from a first portion of the lateralacceleration signal, whereby the first portion of the signal is measuredduring a longitudinal acceleration a₀ of the vehicle; a third step ofobtaining a second lateral acceleration value a_(y CS) from a secondportion of the lateral acceleration signal, whereby the second portionis measured when the longitudinal acceleration has ceased and thevehicle is travelling at constant speed; a fourth step of measuring thelongitudinal acceleration a₀ associated with the first portion of thelateral acceleration signal; a fifth step of determining a toe angle αof the vehicle wheel according to the following relationship:$\alpha = {{\arcsin\left( \frac{a_{Y_{Acc}} - a_{Y_{CS}}}{{- a_{0}}{\cos \left( {\arcsin \left( \frac{a_{Y_{CS}}}{g} \right)} \right)}} \right)} \cdot \frac{180}{\pi}}$where g is the gravitational acceleration constant.
 2. The methodaccording to claim 1, further comprising a sixth step of determining acamber angle γ, in degrees, of the vehicle wheel according to thefollowing relationship:$\gamma = {{\arcsin \left( {- \frac{a_{Y_{CS}}}{g}} \right)} \cdot {\frac{180}{\pi}.}}$3. The method claim 1, wherein the accelerometer mounted on the vehiclewheel is further used in the fourth step of measuring the longitudinalacceleration a₀.
 4. The method of claim 1, wherein the accelerometer ismounted on a wheel rim of the vehicle wheel.
 5. The method according toclaim 4, wherein the accelerometer is mounted on a side face of thewheel rim, close to a rotation axis of the wheel.
 6. The method of claim1, wherein the accelerometer is part of a tyre pressure monitoringsystem.
 7. The method of claim 1, comprising a further step of removingerrors from the lateral acceleration signal, wherein the errors are dueto one of internal misalignment within the accelerometer and to amounting misalignment of the accelerometer.
 8. The method of claim 1,comprising a further step of filtering the lateral acceleration signal.9. The method of claim 8, wherein the lateral acceleration signal isfiltered using a low pass filter or a bandpass filter.
 10. The method ofclaim 1, wherein the second step of obtaining the first lateralacceleration value a_(y Acc) provides taking an average value of thefirst portion of the lateral acceleration signal.
 11. The method ofclaim 1, wherein the third step of obtaining the second lateralacceleration value a_(y CS) comprises taking an average value of thesecond portion of the lateral acceleration signal.
 12. The method ofclaim 1, wherein steps 1 through 5 are repeated at least once, todetermine several values for toe angle, and the method further comprisesa step of calculating an average toe angle from the several toe anglevalues.
 13. The method of claim 2, wherein steps 1 through 6 of themethod are repeated at least once in order to determine several valuesfor camber angle, and the method further comprises a step of calculatingan average camber angle from the several camber angle values.